Capacitive sensing is a technology based on capacitive coupling which takes human body capacitance as an input. The capacitive touch sensor has been widely used in smart phones, tablets and even in the IT displays up to 23 inches, e.g. Notebooks, laptop trackpads, digital audio players, computer displays, all-in-one PCs, with the multi-touch features.
More and more design engineers are selecting capacitive sensors for their versatility, reliability and robustness, unique human-device interface and cost reduction over mechanical switches.
Capacitive sensors detect anything that is conductive or has a dielectric different than that of air. While capacitive sensing applications can replace mechanical buttons with capacitive alternatives, other technologies such as multi-touch and gesture-based touch screens are also premised on capacitive sensing.
Capacitive sensors are constructed from many different media, such as copper, Indium Tin Oxide (ITO) and printed ink. Copper capacitive sensors can be implemented on Printing Circuit Boards (PCBs) as well as on flexible material. Indium Tin Oxide allows the capacitive sensor to be up to 90% transparent for one layer solutions, such as touch phone screens.
In the industry of resistive touch panels, pen writing has been used for many years. The most critical part of a resistive touch panel is the reliability issue. The resistive film is subjected to worn-out problems after intensive use. The resistive touch panel provides the writing experience close to the writing habit of people, and the tip of the pen can be small enough to have relatively high writing resolution.
In the meantime, the technique of the projected capacitive touch panel, which measures the variation of capacitance where the fingers are touching, also advances.
FIG. 1A and FIG. 1B schematically show the structures of conventional two-dimensional sensor arrays (110, 120). In consideration of coordination accuracy of touched locations, the touch sensors often come with two-dimensional sensor arrays, including Double-sided Indium Tin Oxide (DITO) or Single-sided Indium Tin Oxide (SITO). The size of the sensor element from the sensor array is about the fingertip size (5-8 mm) The patterns of the sensor elements are mostly the bar shape, the diamond shape or other polygon shapes. For example, FIG. 1A shows that the pattern of the sensor elements (118, 116) in a two-dimensional sensor array 110 is the bar shape, and the two-dimensional sensor array 110 includes a bottom layer 112 and a top layer 114. FIG. 1B shows that the pattern of the sensor element 122 in a two-dimensional sensor array 120 is the diamond shape.
In general, the two-dimensional sensor array constructed as a matrix-like or keyboard-like structure has less constraint on the trace routing and provides better touch accuracy when compared with the one-dimensional sensor array for multi-touch applications. However, the two-dimensional sensor array costs higher than one-dimensional sensor array in manufacturing.
To have a better Signal to Noise Ratio (SNR) measurement for the finger identification in the traditional sensor array, the area touched by the finger cannot be too small, and the required diameter of the area touched by the finger is about 6 to 9 mm. The required area is relatively large, and thus it is difficult to do the sophisticated pen writing on the capacitive touch screen, especially for the Chinese characters.
FIG. 2 schematically shows the perspective view of another conventional capacitive touch display incorporating a digitizer at the backside. The capacitive touch display 200 includes a capacitive touch panel 202, a thin film transistor liquid crystal module (TFT LCM) panel 204, and a digitizer panel 206. The conventional capacitive touch display with an additional digitizer or an active writing pen provides a pen writing function, but needs extra cost.
Thus, the conventional capacitive touch display suffers from the following drawbacks: (1) the cost is then increased dramatically; (2) the specific digitizer pen is required; (3) the complex mechanical design is required to avoid the signal interference; and (4) the entire device gets thicker.
Therefore, it is desirable to create a capacitive touch sensor to resolve the above-mentioned issues.